An infrastructure-based wireless network typically includes a communication network with fixed and wired gateways. Many infrastructure-based wireless networks employ a mobile unit or host which communicates with a fixed base station that is coupled to a wired network. The mobile unit can move geographically while it is communicating over a wireless link to the base station. When the mobile unit moves out of range of one base station, it may connect or “handover” to a new base station and starts communicating with the wired network through the new base station.
In comparison to infrastructure-based wireless networks, such as cellular networks or satellite networks, ad hoc networks are self-forming networks which can operate in the absence of any fixed infrastructure, and in some cases the ad hoc network is formed entirely of mobile nodes. An ad hoc network typically includes a number of geographically-distributed, potentially mobile units, sometimes referred to as “nodes,” which are wirelessly connected to each other by one or more links (e.g., radio frequency communication channels). The nodes can communicate with each other over a wireless media without the support of an infrastructure-based or wired network. Links or connections between these nodes can change dynamically in an arbitrary manner as existing nodes move within the ad hoc network, as new nodes join or enter the ad hoc network, or as existing nodes leave or exit the ad hoc network.
Wireless media are inherently volatile and channel quality of a wireless communication link between a transmitter node and a receiver node can vary considerably. The channel quality of a wireless link depends on radio propagation parameters such as path loss, shadow fading, multi-path fading, co-channel interference from other transmitting nodes, sensitivity of the receiver node, available transmitter power margin, and the like. To help ensure a certain level of performance for data communications between nodes, Qualities of service (QoS) procedures are often implemented. QoS procedures take place at several communication layers in a communication protocol stack. For example, at the physical layer, QoS is synonymous with signal-to-interference-and-noise ratio (SINR) or bit error rate (BER) at the receiver of each user. At the data link control (DLC) layer and medium access control (MAC) layer, QoS is usually expressed by packet error rate (PER), minimum achievable data rate and maximum tolerable delay guarantees for users. At higher layers, QoS can be perceived as certain data throughput, delay, delay jitter guarantees, or in terms of fairness in rate allocation. In multi-hop networks, QoS has meaning at the network layer in terms of end-to-end bandwidth/delay guarantees.
To meet QoS requirements, many modern wireless communications systems employ link adaptation techniques, sometimes referred to as adaptive transmission techniques or adaptive modulation and coding (AMC) techniques, to improve throughput or data transmission rates (bits/sec) while maintaining an acceptable bit error rate (BER) at the receiver node regardless of link quality. To implement link adaptation techniques, nodes can dynamically adapt one or more transmit parameters depending on integrity and quality of the channel or link between two nodes. For example, a transmitter node can use transmission feedback information (e.g., received signal quality) to dynamically adapt or select one or more transmit parameters to “match” the channel conditions on the radio link as those conditions change. By exploiting channel information present at the transmitter node, an optimal combination of transmit parameters can be selected so that data throughput (i.e., transmission rates) and system capacity are improved or optimized while acceptable bit error rates (BERs) are achieved. At the physical layer, link adaptation technologies can be used to adjust transmit parameters such as transmission data rate, transmission power, modulation level, symbol rate, coding rate and other signal and protocol parameters to mitigate fluctuation in link quality and to maintain acceptable link quality. A number of different strategies can be used for selecting particular transmit parameters from a set of available transmit parameters.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.